Gas discharge lamps can be operated most efficiently by AC (alternating current) power at a relatively high frequency (on the order of 35 KHz (kilohertz). However, line AC power is supplied by utility companies at low frequencies of around 50 or 60 Hz (hertz). To obtain high efficiency operation of the lamps, the AC power at the first low frequency is converted to AC power at a second high frequency.
The conversion of the AC power from one frequency to another is accomplished by a ballast circuit. The AC power at the first low frequency is rectified into DC (direct current) power, and then stored as energy in a relatively large electrolytic capacitor. The energy stored in the electrolytic capacitor is then chopped by an inverter into AC power at a second high frequency.
In this kind of circuit, whenever the voltage of the line AC power is greater than the voltage stored in the electrolytic capacitor, a relatively large surge of current passes into the electrolytic capacitor, causing the line current drawn to be "peaky". This circuit thus has a poor power factor.
One solution is to place a floating voltage supply in series with the incoming line to the capacitor. Such a supply presents several problems. First, the voltage on the supply must be controlled so as to match the voltage on the electrolytic capacitor, otherwise the waveform of the power drawn from the line will be distorted. Second, the impedance of the supply must be adjustable so as to control the amount of power drawn from the power line. If not, the inverter will either produce too much power or there will be little correction of the power factor. Finally, the source of the power for the floating voltage supply must be stable and have low impedance.
A circuit which provides a floating voltage supply in series with the incoming line and has an adjustable impedance and an adjustable voltage level is thus desirable.